The present invention relates to a system that encrypts and decrypts signals, and transmits and receives the encrypted signals, more particularly to a system, such as a packet transmission system, in which the encrypted signals are transmitted intermittently.
One well-known type of encrypted transmission system has the general structure shown in FIG. 8, comprising a first device 220 and a second device 280 linked by a communication channel 300. The communication channel 300 may be a wireline link comprising copper wire, optical fibers, or the like, or a wireless link comprising radio waves, infrared beams, or the like.
The first device 220 has a transmitting section and a receiving section. The transmitting section includes a scrambling circuit 221 that scrambles a transmit signal A to obtain a scrambled signal CA, a pseudo-random pattern generating circuit 222 that supplies a pseudo-random pattern RA1 to the scrambling circuit 221, and a transmitting circuit 223 that transmits a modulated signal MCA, modulated by the scrambled signal CA. The receiving section includes a receiving circuit 230 that receives and demodulates a modulated signal MCB to obtain a scrambled signal CB, a descrambling circuit 231 that descrambles the scrambled signal CB to obtain a receive signal B, and a pseudo-random pattern generating circuit 232 that supplies a pseudo-random pattern RA2 to the descrambling circuit 231.
The second device 280 also has a transmitting section and a receiving section. The transmitting section includes a scrambling circuit 281 that scrambles a transmit signal B to obtain a scrambled signal CB, a pseudo-random pattern generating circuit 282 that supplies pseudo-random pattern RA2 to the scrambling circuit 281, and a transmitting circuit 283 that transmits the modulated signal MCB, which is modulated by the scrambled signal CB. The receiving section includes a receiving circuit 290 that receives and demodulates the modulated signal MCA to obtain a scrambled signal CA, a descrambling circuit 291 that descrambles the scrambled signal CA to obtain a receive signal A, and a pseudo-random pattern generating circuit 292 that supplies pseudo-random pattern RA1 to the descrambling circuit 291.
When signal A is transmitted from the first device 220 to the second device 280, the scrambling circuit 221 uses the pseudo-random pattern RA1 supplied by the pseudo-random pattern generating circuit 222 to alter the contents of signal A in a seemingly random fashion, typically by taking the exclusive logical OR of corresponding bits of A and RA1. As a result, if the modulated signal MCA is intercepted by a third party, the intercepted signal is unintelligible. The descrambling circuit 291 uses the same pseudo-random pattern RA1, supplied by the pseudo-random pattern generating circuit 292, to perform the reverse alteration on the scrambled signal CA (typically by performing another exclusive logical OR operation), thereby obtaining the original signal A.
When signal B is transmitted from the second device 280 to the first device 220, it is similarly scrambled and descrambled, using pseudo-random pattern RA2, which may differ from pseudo-random pattern RA1.
In the system in FIG. 8, the pseudo-random patterns RA1, RA2 are hard-wired into the pseudo-random pattern generating circuits, which are typically manufactured in large quantities. Moreover, the pseudo-random patterns are of finite length, and repeat cyclically. Under these conditions, it is difficult to ensure that an intercepted transmission cannot be descrambled by the intercepting party, who may be in possession of equipment with a similar pseudo-random pattern generating circuit. The only defense is to use a very long pseudo-random pattern, but this requires a comparatively large and therefore expensive pseudo-random pattern generating circuit, and leads to difficulties in maintaining synchronization between the pseudo-random patterns generated in the first and second devices 220, 280.
A known solution to these problems is given in Japanese Unexamined Patent Application No. 05-007202, which discloses an encrypted transmission system that is both simpler and more secure. In place of the pseudo-random patterns employed in FIG. 8, this system uses signal A to encrypt signal B, and signal B to encrypt signal A.
Referring to FIG. 9, this system comprises a first device 200 and a second device 260 linked by a communication channel 300. The transmitting section of the first device 200 includes a converter 201 that uses a received signal B′ as an encryption key to convert a transmit signal A to an encrypted signal CA, and a transmitting circuit 203 that converts the encrypted signal CA to a modulated signal MCA for transmission on the communication channel 300. The receiving section includes a receiving circuit 210 that receives and demodulates a modulated signal MCB and outputs an encrypted signal CB, a deconverter 211 that decrypts the encrypted signal CB to obtain the receive signal B′, and a memory 202 that stores the transmit signal A sent to the converter 201 and supplies the stored signal A as a decryption key to the deconverter 211.
The second device 260 has a similar structure. Its transmitting section includes a converter 261 that uses a received signal A′ as an encryption key to convert a transmit signal B to an encrypted signal CB, and a transmitting circuit 263 that converts the encrypted signal CB to a modulated signal MCB for transmission on the communication channel 300. The receiving section includes a receiving circuit 270 that receives and demodulates a modulated signal MCA and outputs an encrypted signal CA, a deconverter 271 that decrypts the encrypted signal CA to obtain the receive signal A′, and a memory 262 that stores the transmit signal B and supplies it as a decryption key to the deconverter 271.
Because it uses the receive signals A′ and B′ as encryption keys, and the transmit signals A, B as decryption keys, this system does not require separate circuits for generating pseudo-random patterns. A high level of security is provided, even if a simple encryption procedure is used, because the encryption and decryption keys are constantly changing. Encryption by the exclusive logical OR operation, for example, provides better security in FIG. 9 than in FIG. 8.
The system in FIG. 9 has the disadvantage, however, of requiring synchronization between the transmit signals, so it cannot be used when A and B are intermittent signals.
If the first device 200 encrypts the transmit signal A by performing exclusive logical OR operations, for example, then for each bit of A, the converter 201 uses a corresponding bit of the receive signal B′. If the second device 260 transmits signal B intermittently, the required bits of the receive signal B′ may not be available when they are needed. Similarly, if signal A is not transmitted continuously (A1, A2, A3, . . . ), the receive signal A′ may not be available when needed for encrypting transmit signal B.
The system shown in FIG. 9, accordingly, cannot be used in packet communication systems, which include the numerous systems employing the internet protocol (IP).